This site allows students to practice math facts in a timed environment. There is a timer on the screen, and after students have done 20 math problems, the score on those problems is displayed.
Purplemath contains practical algebra lessons demonstrating useful techniques and pointing out common errors. Lessons are written with the struggling student in mind, and stress the practicalites over the technicalities. Links and other resources also available.
There are so many math applets on this site for teachers and students to use. There are applets from adding fractions to AP Calculus and AP Statistics. Check them out.